Thanks all for the suggestions!
I am a PS6 newbie (though been working w/ PS Elements for a few months).
I recall reading about Layer Masks in a tutorial somewhere (on this site?).
If I understood correctly, creating this 1-time layer mask would enable me to place multiple images (layers) side-by-side to make up the "full" image I intend to have, and the mask would cause all the images in the layer set to be clipped according to the mask, based on where they were placed (if the mask is an arch shape across the top, the images would all have a different portion of that curve)?
Forgive the terminology that I'm probably mis-using!
An example of what I'm talking about is
http://www.hyatt.com. Depending on the page you view, the arch along the top is either 1 or 2 images, clipped to form that arch in a seamless fashion.